Liquid crystal display devices utilize optical (refractive) anisotropy and dielectric anisotropy of liquid crystalline compounds. Viewing from the aspect of display mode, display devices of twisted nematic (TN) type, super twisted nematic (STN) type, dynamic scattering (DS) type, guest,host (GH) type, or DAP type organized according to their electrooptical effect are known. From the aspect of driving mode, display devices of static driving mode, time shearing addressing mode, active matrix driving mode, or two-frequency addressing scheme are known. Liquid crystal substances used for the display devices are required to exhibit a liquid crystal phase in a wide temperature range and to be stable against heat, light, moisture, air, electric field, and electromagnetic radiation.
However, since it is impossible to satisfy all of such requirements by a single compound at present, it is a current situation that several, or in some cases more than twenty liquid crystalline compounds are mixed and used as liquid crystal compositions. Accordingly, the liquid crystalline compounds have come to be required to exhibit excellent miscibility each other. Recently in particular, since display devices are used in many instances in sever environments such as at cryogenic temperatures, improvement of the compounds in the miscibility at low temperatures has come to be required.
With the increase of demand for display devices of high quality, requests for liquid crystal composition having an improved response speed to the change in electric field, and steepness have been increased. In order to fulfil the requirements, it is required to use liquid crystal materials having a low viscosity and a large dielectric anisotropy value as component of liquid crystal compositions.
In order to increase the response speed of liquid crystal compositions, it is effective to make the thickness (d) of liquid crystal cells small (thin) in addition to satisfying the requirements described above. In this case, however, it is necessary to increase optical anisotropy value (.DELTA.n) (to use liquid crystalline compounds having a large .DELTA.n as component) for the reason described below. That is, excellent display contrast and wide visual angle are found as other characteristics required of display devices. In order to achieve this end, it is necessary to maintain the product of .DELTA.n and d (.DELTA.n.multidot.d) at a certain value, and thus, when d is made small according to the measures described above, it is necessary to increase .DELTA.n.
For originating liquid crystalline compounds having a large .DELTA.n, it is sufficient, for instance, to introduce ethynylene group into a liquid crystal molecule, and it is known by means of the compounds expressed by the formula (13) (Japanese Patent Publication No. Hei 3-29051) and compounds expressed by one of the formulas (14) to (16) (DE 4027458 and WO 90-13610) that the compounds can be changed into ones having a large value even in dielectric anisotropy (.DELTA..epsilon.) by altering the ethynylene group into the one having an electron attractive group, for example, CN, Cl, CHF.sub.2, or CF.sub.3 as substituent. ##STR3## wherein R represents an alkyl group.
However, as to the compounds expressed by the formula (13), their electric properties are not disclosed at all in the Japanese Patent Publication No. Hei 3-29051. Compounds expressed by one of the formulas (14) to (16) are narrow in the temperature range of liquid crystal phase, and they can not be said to exhibit a sufficiently large value of either .DELTA.n and .DELTA..epsilon. besides.
An object of the present invention is to solve the defects in the conventional technology. Another object of the present invention is to provide conjugated nitrile derivatives, novel liquid crystalline compounds having a sufficiently large .DELTA.n and a large .DELTA..epsilon., being excellent in miscibility with other liquid crystalline compounds, having a low viscosity, and being chemically and physically stable; to provide liquid crystal compositions comprising the nitrile derivative; and to provide liquid crystal display devices fabricated by using the liquid crystal composition.